Electronically controlled variable mode respirator



United States Patent [72] Inventor Dudley George Foster Barnet, England [21] Appl. No. 617,305

[22] Filed Feb. 20, 1967 [45] Patented Aug. 11, 1970 [73] Assignee W. Watson & Sons Limited Barnet, England a British company [32] Priority Feb. 24, 1966 [33] Great Britain [54] ELECTRONICALLY CONTROLLED VARIABLE MODE RESPIRATOR 11 Claims, 4 Drawing Figs.

[52] US. Cl. 128/145.6, 128/ 145.8

[51] Int. Cl. A62b 7/04 [50] Field ofSearch 128/145.5-

15.8, Resp. Digest {56] References Cited UNITED STATES PATENTS 2,830,580 4/1958 Saklad et a1 128/145.8

2,924,215 2/1960 Goodner 128./145.6

3,033,195 5/1962 Gilroy etal.. 128/145.8

3,101,708 8/1963 Perryetal.... l28/145.5

3,251,359 5/l966 lsmach l28/145.8 3,266,488 8/1966 Andreasen 128/145.5

FOREIGN PATENTS 1,017,808 10/1952 France 128/1458 847,280 9/1960 Great Britain [ZS/145.6

OTHER REFERENCES Greer, F.J.R. et al, Brit. Journal of Anesthesia (1958),30,32,(PP.32-36 Relied Upon) copy in GR.335 Rochford, .l. ,et a1, Brit. Journal of Anesthesia (1958),30,23,(pp.23-31 relied upon) copy in GR.335 The Lancet, May 29,1965, (pp-1 145-1 147)copy in GR.335 Primary Examiner Richard A. Gaudet Assistant Examiner- Kyle L. Howell Attorney Watson, Cole Grindle and Watson ABSTRACT: A medical respirator includes three bellows (20,21,22) mechanically linked together and a manually controlled bag 26. Gas may be supplied to and removed from a patient on open or closed circuit operation or on operation of the manual bag.

The inspiration and expiration cycle is by an electronic timing circuit, pressure responsive means (89,90) and volume sensitive means (100). The cycles will be determined by time unless the pressure or volume of gas fed to or from the patient reaches predetermined limits before the expiration of the time allowed.

78 i I l l MON/702 Patented Aug. 11; 1910 Sheet aunt,

um, which ELECTRONICALLY CONTROLLED VARIABLE MODE RESPIRATOR The present invention relates to a medical ventilator or respirator.

The present invention consists in a respirator for use with an anaesthetic and in the treatment of respiratory insufficiency,

. comprising inspiration and expiration bellows mechanically linked so that they may expand and contract in unison and wherein an anaesthetic and/or other gas supplied to the respirator when in use, is passed from the inspiration bellows to a patient as directed by an electrically operated inspiratory control valve, and gas exhaled by the patient flows through an electrically operated expiratory control valve, said valves being alternately closed by an electronic circuit adapted to control the termination of the inspiration and expiration periods of a respiratory cycle by closure of the inspiration and expiration control valves respectively under the influence of a predetermined elapsed time or the attainment of a predetermined pressure or volume of gas passing to or from the patient, and means being provided to select the required parameter for effecting said control.

According to a preferred form of the invention the respirator comprises an expiratory or negative bellows mechanically linked to two other bellows, a center or driving bellows and an auxiliary bellows, the three bellows expanding and contracting in unison, and wherein selection means operated by a common control shaft and knob enable gas supplied to the respirator to be fed in a first selected position to the center bellows, and in a second selected position to both of the said other bellows and in a third selected position to a manual bag and to a patient. and wherein, in the said first and second positions, the manual bag is isolated from the gas circuit and the electronic circuit is adapted to include an overriding trigger device which causes an expiration period to be terminated rapidly, and an inspiration period to be initiated, should a patient make an attempt to inspire during such expiration period. Gas for a patient or a driver gas is fed in the first and second selected positions to the center bellows from, for example, a cylinder of gas under pressure to extend the center bellows when the inspiratory control valve is closed. The three bellows when the inspiratory control valve is closed. The three bellows are connected to a movable platform on which torsional spring means operate to provide a force to return the bellows to a collapsed condition when the inspiration control valve is open. Gas for a patient passes in the first selected position from the center bellows, and in the second selected position from the auxiliary bellows to a face mask or other means for supplying gas to a patient, the gas exhaled by the patient flowing through the expiration control valve and negative bellows. In the said first and second selected positions means are provided for the introduction of the electrical contacts of sensors responsive respectively to the volume and pressure of gas to and from the patient, the closure of at least one of the said electrical contacts decreasing the effective resistance value of one of two resistance chains to cause the electronic circuit to terminate a period of the respiratory cycle, such periods, in the absence of such introduction and closure, being respectively terminated at the end of periods of time the duration of which is controlled by the respective ohmic resistance of said chains.

Means are also provided for producing a negative (below atmospheric) pressure to assist in extracting gas from the lungs of a patient, said means comprising a negative bellows, of greater stroke volume than that of either of the two other bellows, in combination with a variable orifice for adjusting the negative pressure between desired limits. Preferably a safety valve is also provided so as to avoid the possibility of excessive negative pressure being developed to the detriment of a patient.

One embodiment of the invention will now be described by way of example and with reference to the accompanying drawings in which:

FIGURE l is a diagram ofthe gas circuit employed in a ventilator according to the invention and also shows some mechanical parts;

FIGURE 2 is a simplified diagram of the electronic curcuit and electrical switching for controlling gas flow to and from a patient;

FIGURE 3 is a circuit diagram of a remotemonitor unit for use with the ventilator of FIGURE 1, and

FIGURE 4 is a circuit diagram of part of FIGURE 2 using an alternative switching arrangement.

Referring to FIGURE 1 a ventilator comprises a substantially cubical framework fabricated from metal of strip, angle and channel form, parts 1, 2, 3, 4, 5, 6, and 7 of which are indicated at the top of the drawing. The faces of the framework are clad with panels of sheet metal to form the housing of the ventilator, the panels having suitable apertures for the controls, indicators and gas inlets and outlets. Two rear upright members 2 and 7 of the framework respectively carry bearings 8 and 9 in which a rod 10 is free to rotate. Firmly secured to the rod are two parallel arms 11 and 12 at the extremities of which are two bearings 13 and 14 which carry a platform 15. A helical spring 16 surrounds the majority of the length of the rod 10, one end of the spring being engaged with the arm I! and the other end being engaged with a worm wheel 17, through the center of which passes the rod 10, mounted on the upright member 7. A shaft 18 embodying a worm gear in mesh with the worm wheel 17 is also mounted on the upright 7, rotation of said shaft 18 turning the worm wheel 17 on its mounting and giving a torsional stress to the spring 16. A platform, indicated by the broken line 19, is supported within the framework and carries three cylindrical bellows, a negative or expiratory bellows 20, a central bellows 21 and an'auxiliary bellows 22, whose upper ends are attached to the platform 15. Rotation of the worm wheel 17 is made in a direction such that the torsional stress tends to lower the platform 15 and collapse the bellows. The degree of such rotation will therefore control the force tending to collapse the bellows, whiledue to the length of spring involved such force may be accurately set and be substantially constant over the range of movement of the arms 11 and 12.

The negative bellows 20 has an internal projection of inverted top hat configuration depending from its upper face, as shown by the broken line 23, such that in the collapsed condition of the bellows the residual volume is very small. The maximum stroke volume of the negative bellows is two thousand cubic centimeters, the other two bellows each having a maximum stroke volume of one thousand four hundred cubic centimeters, to enable a negative (below atmospheric) pressure to be developed.

The ventilator may be used with a gas supply to a patient on open circuit, closed circuit or manual, selection of themethod of use being by a single control on the ventilator. In' the first two cases the ventilator is operated on either time, pressure, or volume cycling or a combination of these by means of two alternately energised electrically operated valves, an inspiration valve 24 and an expiration valve 25, while in the latter case the respiratory cycle is controlled by operation of a manual bag 26. l

The gas circuit is shown in FIGURE l and includes seven gas ports, a first closed circuit port 27, a second closed circuit port 28, a driving gas port 29, a patient gas port 30, a manual bag port 3], a patient inspiration port 32 and a patient expiration port 33, connected by piping to the bellows by means of eight poppet type valves Vl-V8 (shown open for clarity) and a gas distribution valve 34. The latter and the eight valves Vl- V8 are actuated by a common control shaft 200 to which is attached a control knob 202 for manual selection of the mode of usage. i.e. manual, open circuit or closed circuit. The valves Vl-V8 are each held in the closed position by an external coil spring (not shown) surrounding the valve stem protruding from the valve casing and are opened as required for the selected mode of usage by eight cams on the control shaft actuating rocker arms engaged with the valve stems. The valves Vl-VS have ports designated A, B, C, D, E, F, G, H, I, K, L and M, some of which are common to two valves. The gas distribution valve 34 is in the form of a cylindrical bore 35 containing a rotor 36 coupled to the control shaft and formed from a solid cylinder of appropriate diameter and length. Two parallel holes 37 and 38 traverse two diameters of the rotor, which is also relieved throughout its length to form two diametrically opposed flat surfaces 39 and 40 parallel to each other and to the axis of the rotor. Four ports S, P, R and N enter the cylindrical bore 35 and are arranged so that, in one position of the rotor, ports S and P are joined by hole 37 and ports R and N are joined by hole 38 to permit gas flow between ports S and P and between ports R and N. Rotation of the rotor, clockwise or counterclockwise, first misaligns the holes and ports, then masks the ports, and finally again unmasks the ports, a gas passage between ports S and R then being provided by part of the cylindrical bore 35 and a flat surface 39 or 40 and another gas passage being provided by another part of the cylindrical bore 35 and the other flat surface 40 or 39. Such gas passages are not only formed when the flat surfaces directly face the ports but are also formed when one edge of a flat surface unmasks a pair of ports and they remain until the appropriate edge of the flat surface covers the ports, i.e. there is a considerable angular range of rotation of the rotor during which such gas passages are effective. Advantage is taken of this to give the control knob the required three mode positions, determined by normal detent means, the shaft rotating 120 between each position. This 120 degrees rotation between positions gives an added advantage in that the cams operating valves V1 V8 may have a lower rate of lift than if 90 rotation between positions were employed.

When the ventilator is in use a patient inhales and exhales via a port 41 and is connected to the inspiration port 32 and expiratory port 33 ofthe ventilator by flexible tubes 42 and 43 respectively, the junction of the tubes incorporating a spill valve 44. The spill valve 44 is lightly loaded to the closed position by a spring 45 and permits gas in the tubes to exhaust to atmosphere via a port 44A when pressure in the circuit exceeds a predetermined adjustable level. Provision is made, by means not shown, to lock the spill valve 44 inoperative in the closed position to prevent such exhausting to atmosphere. Gas supply to the patient is connected to port 30 from a source incorporating a flow control at a pressure between for example, 3 and P.S.l. through a flow meter, as the ventilator is of the minute-volume divider type.

During manual operation of the ventilator the distribution valve 34 is in the position shown, valves V1, V3, V6 and V7 are open, and the manual bag 26 is connected to its port 31, the spill valve 44 being unlocked to allow it to function. The opening of valves V3 and V7 permits, as valve 24 is either completely or periodically unenergised and open, any gas contained within bellows 21 and 22 to escape to atmosphere as the bellows contract under the influence of the spring 16, any gas in bellows venting to atmosphere through port 28.

Gas flows from the port 30 to port P via pipe 46, through the passage between ports P and N in the distribution valve 34, out of port N to port G of open valve V6 via pipe 47, out of port H of valve V6 to port A of open valve VI via pipe 48, and out of port B of valve V1 to inflate the manual bag 26 via pipe 49 and port 31. Gas also passes from port C of open valve V1 via pipe 50 to a center zero gas pressure gauge 51 and to port 32 via pipe 52 from whence it travels by tube 42 to the spill valve 44 and to the patient port 41. Gas will not flow into tube 43 and via port 33 into pipe 53, through a tidal volume meter 54, pipe 55, solenoid expiration valve (which is either completely or periodically unenergised and open) and pipe 56 to port B of valve V5, as this latter valve is not open. Gas also will not flow from port C via pipe 57, flow rate control valve 58, pipe 59, diaphragm valve 60 and pipe 61 to port M, as neither valve V4 nor valve V8 are open. Constriction ofthe inflated manual bag 26 by manual pressure will inflate the patient, some gas escaping to atmosphere via the spill valve 44, the pressure used for such inflation being shown on the pressure gauge 51. Subsequent relaxation of the manual pressure on the bag 26 will allow this to re-inflate, the gas in the patient 5 lungs being expelled by the elastic recoil ofthe chest wall, a

minority of the expelled gas returning along the flexible tube 42, giving an unavoidable degree of rebreathing of gas, and the majority exhausting to atmosphere through the spill valve 44.

During manual operation the electrical supply of the ventilator need not be switched on, but when employing either of the other two modes of use, electrical power is required for an electronic circuit used to alternately energise the electrically operated solenoid valves 24 and 25 which control the respiratory rhythm or cycle comprising an inspiration period and a subsequent expiration period. Should electrical power be applied to the electronic circuit during manual operation, valves 24 and 25 will be alternately energised but will have no harmful affect. If required, an electrical interlock may be arranged to prevent such application of power when manual operation is selected, but without this facility it is possible to change from manual to another mode of operation controlled by the electronic circuit without interruption of the respiratory rhythm.

When a closed gas circuit is in use the distribution valve 34 has its rotor 36 turned so that hole 37 is aligned with ports S and P and hole 38 is aligned with ports R and N and the spill valve 44 is locked to prevent gas exhausting to atmosphere. Valves V2, V3, V5 and V8 are open, and ports 27 and 28 are linked by a tube 62, a branch 63 from which feeds a gas purifier 64 (a C0 absorber) to which a reservoir 65 having flexible walls is connected by a tube 66. The reservoir 65 is capable of expanding when a gas is received therein with substantially no increase in gas pressure due to the expansion of the container walls. A driving gas, such, as for example, compressed air, is applied to the port 29 and gas for the patient to the port 30 as for manual operation. The flow of the driving gas is also controlled by external means not shown, the patient gas initially charging the ventilator and thereafter making good any losses due to absorption by the patient and leakage.

The driving gas passes from port 29 via pipe 67 to port R of the distributor valve 34, out of port N via pipe 47 to port G of open valve V2, out of port F via pipe 68 to inlet 69 of bellows 21, and from the outlet 70 of bellows 21 to the inspiration valve 24 by pipe 71. During the expiration period valve 24 is energised and closed and the bellows 21 will be expanded by the driving gas, raising the platform 15 and so expanding the other two bellows. During the following inspiration period the valve 24 is de-energised and open, allowing gas from the bellows 21 to pass via pipe 72 to port I and thence to atmosphere from port L of open valve V3 and pipe 73. During this inspiration period the spring 16 forces the three bellows towards the contracted condition.

Patient gas from port 30 passes via pipe 46, port P, hole 37, port S, pipes 74 and 75 to port 27, and thence via tube 62, branch 63, gas purifier 64, and pipe 66 to charge the reservoir 65. The remainder of the gas system may be charged with patient gas by, for example, removing tube 62 from port 28, and allowing the machine to cycle a few times with the disconnected end of tube 62 masked by a hand of an operator and then reconnecting tube 62 to port 28. Alternatively the flow rate to the ventilator may first be made high to flush out unwanted air from the gas circuit. In the following description such charge of the system is assumed to have occurred.

During the expiration period the expanding bellows 22 draws gas from the reservoir 65 through the connections described, pipe 76 and a one way valve 77 in an inlet 78 to the bellows 22. At the same time gas within the patient flows into the expanding bellows 20 via the tube 43, port 33, pipe 53, respirometer 54 (showing tidal volume of expired gas), pipe 55 unenergised and open expiration valve 25, pipe 56, ports D and E ofopen valve V5, pipe 79 and an unrestricted inlet 80 of bellows 20. On the inspiratory stroke of the three bellows, with platform 15 descending, gas in bellows 22 is passed through a one way valve 81 in an outlet 82 of bellows 22, pipe 83, ports K and M of open valve V8, pipe 61 diaphragm valve 60, flow rate control valve 58, which regulates the speed of gas flow to the patient, pipes 57, 50 and 52, port 32 and tube 42 to inflate the patient connected to port 41. Gas does not pass along tube 43 as during the inspiration period the expiratory valve 25 is energised and closed. At the same time gas in bellows 20 is passed out through a one way valve 84 of an outlet 85 of bellows 20 via a pipe 86 to replenish the reservoir 65 from port 28. During this inspiration period the positive (above atmospheric) pressure will be shown on the gauge 51 and will also influence, through pipes 87 and 88, two switches 89 and 90 operated by diaphragms responsive to changes in pressure.

As was earlier stated the stroke volume of the negative bellows 20 is larger than that of the other two and during the expiratory stroke a negative (below atmospheric) pressure would be produced with the system as described. To allow this to be controlled from a negative maximum down to zero (atmospheric pressure) a negative control valve 91 is utilised. The inlet of the valve 91 is connected by a pipe 92 to the pipe 86 and the outlet of the valve 91 is connected by pipe 92A to a one way valve 93 ofa second inlet 94 of bellows 20. The valve 91 takes the form of a variable bleed and has incorporateda valve 91A, spring loaded to the closed position, bypassing the control valve 91 and arranged so that an excessive negative pressure, which might be dangerous to the patient, opens the valve 91A. During closed circuit operation the additional gas required to offset negative pressure in excess of any negative pressure which may be required is therefore drawn from the reservoir 65. Under conditions when no negative pressure is required, the negative control valve 91 is fully open, the one way valve 93 opening as required to admit gas to the bellows 20 to make up the difference in stroke volume of bellows 20 and 22.

The flow rate control valve 58 is of similar construction to the negative control valve 91, but has no bypass valve similar to 91A.

When negative pressure is in use it is not only applied to the patient but to'the tube 42 and pipes in connection with port 32. The diaphragm valve 60 closes under these conditions to prevent valve 81 opening as bellows 22 expand and to reduce the volume of gas that may be drawn towards bellows 20. The negative pressure in use during expiration is thus shown on the gauge 51 and is applied to switches 89 and 90. The reduction in minimum volume of the bellows 20 by the internal projection depicted by the broken line 23 is utilised to establish rapidly the required working negative pressure.

The center bellows 21 incorporates an outlet valve 95 in its upper face. The stem 96 of the valve 95 passes through the platform 15, a spring 97 co-operating with the stem 96 and platform to hold the valve closed. A striker disc 98 is adjustably secured to the external end of the stem 96 and is positioned so that expansion of the bellows beyond a desired limit causes the disc 98 to contact part of the framework, such as parts 3 and 5 or a part attached thereto, and open the valve 95, so allowing egress of driver gas and preventing further expansion ofthe three bellows.

Attached to the platform 15 by a link 99 is a striker bar 100 moving vertically in guides (not shown). A pointer 101 is carried by the bar 100 and portion of the pointer is visible in an aperture 102 in a front panel of the framework. A rod 100A, attached to the striker bar, projects through the top surface of the ventilator housing, to give a readily visible indication of the expansion and contraction of the bellows. Adjustable in a vertical direction in the framework by two control knobs (not shown) are an upper masking plate 103 and a lower masking plate 104 located in a plane behind the pointer 101 and of such width that they overlap the sides of the aperture 102, positional adjustment of the plates 103 and 104 by their control knobs thereby exposing a particular selected area of the aperture 102. Mounted on the upper end of the upper plate 103 are two micro switches, an upper volume micro switch 105 and an upper monitor switch 106 arranged so that, as the striker bar 100 rises under the influence of the expanding bellows, a face 107 will engage the operating member of, and close, the volume micro switch 105. Should the striker bar 100 rise fractionally further a face 108 on the bar will engage the operating member of, and open, the monitor micro switch 106. Similarly mounted on the lower end of the lower masking plate 104 are a further pair of micro switches, a lower volume micro switch 109 and a lower monitor micro switch 110 closed and opened by the faces 111 and 112 respectively when the bar 100 descends as the bellows contract from an expanded condition. Position the masking plates 103 and 104 thus controls, in addition to the aperture size, the point at which the switches 105, 106, 109 and 110 are actuated during expansion and contraction of the bellows. The dimensions of the arrangement are such that when one of the micro switches or 109 is first operated by the striker bar, the pointer 107 is aligned with the end of the associated masking plate 103 or 104 visible in the aperture 102. Switches 106 and therefore operate when the pointer 107 fractionally masks an exposed end of plates 103 and 104 respectively.

For open circuit operation the tube 62 is removed from port 28 and preferably from port 27 also. Patient gas is fed as before to port 30 and no driving gas is applied to port 29. Distribution valve 34 is in the position shown with ports 5 and R connected together, as are ports P and N. Valves V2, V4, V5 and V7 are open, the remainder of the sight being closed. Patient gas, which also acts as a driver gas, flows via pipe 46, ports P and N, pipe 47, ports G and F of open valve V2, pipe 68 and inlet 69 to bellows 21. During an expiration period, inspiration valve 24 is energised and closed and the center bellows 21 therefore expands driving platform 15 upwards and expanding bellows 20 and 22. During this expansion air is drawn into bellows 22 through inlet 78 and one way valve 77 from ports 27 and 29, gas from the patient being passed to the bellows 20 through tube 43, port 33 and the pipes linking the respirometer 54, unenergised and open expiration valve 25, ports D and E of open valve V5 to inlet 80. Negative pressure may be employed as for closed circuit operation to draw gas from the lungs of patient or assist in gas explusion therefrom. The negative control valve 91 in this case allows the passage of air from port 28 to the bellows 20. During the following inspiration period the valves 24 and 25 are in the open and closed positions respectively and the bellows are contracted by the spring 16 and the following action occurs. Gas in be!- lows 20 is exhausted to atmosphere through one way valve 84, outlet 85, pipe 86 and port 28. The air drawn into bellows 22 during the expiration period is passed back to atmosphere via one way valve 81, outlet 82, pipe 83, ports K and L of open valve V7 and pipe 73. Patient gas in the bellows 21 passes from outlet 70 via pipe 71, valve 24, pipe 72, ports 1 and M of open valve V4 and thence by pipe 61 and the route described for closed circuit operation to the patient.

As before, the outlet valve 95 opens at a predetermined expansion limit, the pressure gauge 51 indicates the positive and negative pressure in use and which are applied to the pressure sensitive switches 89 and 90, while diaphragm valve 60 allows gas to flow from pipe 61 to pipe 59 whilst preventing extraction of gas through pipe 59. To perform this function (in both closed and open circuit) the diaphragm valve has port 60A (to which pipe 59 is attached) closed by the front face of a flexible diaphragm 60B, pipe 61 admitting gas to a compartment containing the port 60A and bounded by the walls of the valve and the diaphragm 608. Gas under pressure entering this compartment from pipe 61 displaces the diaphragm, unmasks the port 60A and so allows the gas to enter pipe 59. Negative pressure applied via pipe 59, however serves only to close the port 60A more securely, as the back of the diaphragm 60B is at atmospheric pressure due to the aperture 60C in the valve wall.

The flow rate control valve 58 comprises a body having a cylindrical bore with an upper and lower port. Within the bore is a hollow cylindrical rotor having a shaft projecting through the body for its rotation, the interior of the cylindrical rotor being open to the bore containing the lower port and part of the cylindrical wall of the rotor masking the upper port, seals being provided to prevent gas leakage between the ports and to atmosphere. in line with the upper port a group of radially disposed holes pierce another part of the rotor wall, rotation of the rotor by the shaft sequentially and accumulatively exposing the holes to the upper port. The diameter of the holes in the rotor wall is varied in conformity with the law (rotation/flow rate) desired, a control knob being fitted to the shaft for manual adjustment of the rate. This method of construction involves a much easier manufacturing process than the more usual method employing either a shaped port or rotor opening or a combination of both. in the flow rate valve 58 the ports are in direct connection with the associated pipes 57 and 59, but in the negative control valve each port leads to a separate compartment having one wall common to both. Pipes 92 and 92A communicate with the lower and upper compartments leading to the lower and upper ports respectively and the common wall of the compartments is pierced by an aperture having a valve seat insert. A valve, the head of which is in the upper compartment and adapted to close the aperture, has its stem passing through the lower wall of the lower compartment, sealing means being provided to prevent the escape of gas. The wall of the compartment through which the valve stem passes is comparatively thick and the hole therein is tapped to receive an externally threaded flanged valve guide bearing. The sealing means comprise a flexible washer of, for example, neoprene, clamped to the outer surface of the valve guide flange and with its inner edge embedded in a groove in the valve stem. The external end of the valve stem is threaded and carries a nut, between which latter and the compartment wall through which the stem passes is a helical spring surrounding the stem. Movement of the nut on the stem towards the head compresses the spring which thus holds the valve in the closed condition. Gas in the upper compartment may be below the pressure of that in the lower compartment, usually at or about atmosphere pressure, and if this pressure difference exceeds the spring pressure, the valve will lift to reduce the pressure difference. Adjustment of the nut position thus controls the pressure difference at which the by-pass valve operates.

The positive pressure switch 90 comprises a short cylindrical compartment having a flexible metal diaphragm for one circular wall. The interior of the compartment is in communication with pipe 88 and the external center ofthe diaphragm is arranged to move a first electrical contact carried on a spring blade. A second electrical contact aligned with first, is carried on a rigid blade, whose position may be altered by rotation of a control shaft and knob to vary the distance between the contacts. The contacts are arranged so that an increase of pressure above atmospheric pressure within the compartment tends to close them, the control knob carrying calibration marks of the positive pressures at which closure of the contacts occur e.g. to 50 cm. H O. The negative pressure switch 89 is ofsimilar construction to the positive pressure switch except that a negative (below atmosphere) pressure within the compartment tends to close the contacts and the control knob carries calibration marks for the negative pressures at which closure ofthe contacts occur e.g. 0 to 30 cm. H2O.

Inspiration valves 24 and expiration valve 25 are identical solenoid operated valves and each consists of two compartments, an upper compartment and a lower compartment, with a common dividing wall, each compartment having an outlet to which pipes may be attached, gas flow being from the upper to the lower compartment. The dividing wall and an opposite wall in the upper compartment are pierced to receive a valve seat and stem respectively in a similar manner to that employed in the by-pass valve 91A, but the valve stem is directly connected to the moving armature ofa solenoid operating coil and is not spring loaded, gravity being used to return the valve to the open position. The valves are mounted in the ventilator with the valve stems vertical, but they will open and close in a normal manner even when degrees out of vertical should the ventilator be tilted. The wall of the upper compartment traversed by the valve stem is comparatively thick and the hole therein is threaded to receive an externally threaded flanged valve guide bearing. The sealing means include a flexible washer, as in the by-pass valve 91A. The solenoid is mounted concentrically with the valve stem and the valve guide serves as a bearing for both the valve stem and the moving armature of the solenoid. There is thus, when the valve is closed, a possibility of gas passing through the bearing and being enclosed between the flexible washer and the valve guide flange. Such trapped gas would tend to prevent the valve opening immediately upon the solenoid being de-energised, and to prevent this a helical groove is provided in the valve stem to establish a passage through which gas may pass freely between the upper compartment and the enclosed space bounded by the face of the valve guide and flexible washer. Closure of the valve is also made slightly faster as little work is done on the gas when the above mentioned enclosed space is enlarged by upward flexing of the sealing washer. An externally adjustable stop is provided in the lower compartment to engage the valve head and prevent the valve opening to such an extent as to impair the magnetic circuit. The moving central iron armature of the solenoid is shorter than the latter and when the valve is closed the lower ends of armature and solenoid coil near to the valve are approximately in line. The remaining space within the solenoid central tube at the upper end is partially filled with an iron slug moveable within the tube by an external adjusting screw to control the size of an air gap in the magnetic circuit when the solenoid is energised. In order to minimise losses and provide rapidly acting valves at least a part of the magnetic circuit, the central armature, is of high permeability material e.g. such as that known under the trade name of Radio Metal The electrical and electronic parts of the ventilators receive power from a low voltage, nominally 6.5 volts battery 113 of nickel iron secondary cells housed within the ventilator, which is of sufficient capacity to run the ventilator for over twenty four hours. A mains driven charger 114 for this battery is included in the ventilator, the output of the charger being automatically controlled and arranged to substantially fully charge the battery 113 from a discharged condition in approximately three hours and automatically reduce to trickle charging at a very low rate when so required by the battery condition.

Due to the controlled output of the charger it may be used to operate the ventilator in the absence of the battery. Both battery 113 and charger 114 are readily removed for servicing, plug and socket connections being used to facilitate such removal.

A simplified circuit diagram of the electronic control for valves 24 and 25 is given in FIGURE 2, and comprises a transistor astable relaxation oscillator directly coupled to Darlington connected driver and power transistors (used as switches) in which collector circuits are the solenoid operating coils 24A and 25A of the two valves 24 and 25.

The relaxation oscillator is a transistorised variation of a Scarrott oscillator, in which an additional cross coupling has been added to give a symmetrical circuit of the multivibrator type having a capacitor coupling the emitter resistors of two transistors. in such a circuit the time constant of the capacitor and one emitter resistor controls the duration of one period and the other emitter resistor and the capacitor control the duration of the following period, the sum of the two periods comprising the time of one cycle. Also, to avoid the use of very large capacitors in obtaining the required long time constants in the cross couplings, direct coupled amplifiers are inserted in each cross coupling, each of said amplifiers also providing the low impedance output suitable for driving a transistor directly connected thereto.

Referring to Figure 2, battery 113 and/or charger 114 supply power to the circuit through an ON/OFF double pole double throw switch [15, shown in the ON position, and which in the OFF position applied a small resistive lead to the battery via the charger 114 and load 116 to prevent excessive gassing due to over-charging when the ventilator is not in use. Transistors T1 and T2 of p.n.p. polarity have their emitters coupled by a capacitor TC. The emitter of transistor T1 is connected to the positive supply through a resistance REl comprising serially connected resistor R1, pre-set resistor R2 and potentiometer R3 connected as a variable resistor. The emitter of T2 is similarly connected by a resistance RE2 comprising resistor R4, pre-set resistor R5 and potentiometer R6. A direct coupled transistor amplifier 117 using n.p.n. transistors is interposed between the collector of T1 and the base of T2, resistor R7 being the load of T1 and resistor R8 of low resistance being the major part of the load of amplifier 117. A similar amplifier 118 couples the output of T2 to the input of T1, resistors R9 and R10 performing similar functions to resistors R7 and R8 respectively. Resistors R11 and R12 are emitter resistors common to both amplifiers. The output of amplifier 117 provides the input to a Darlington pair of p.n.p. transistors T3 and T4 in whose collector circuit is the operating coil 24A of the inspiration solenoid valve 24. Darlington connected transistors T5 and T6, also of p.n.p. polarity, are similarly driven by amplifier 118, and have the operating coil 25A of valve 25 in their collector circuit. The output stages of amplifiers 117 and 118 give maximum output when their inputs are a minimum and vice-versa, i.e. with T1 non-conducting the output stage of amplifier 117 is conducting hard, switching on T2 and the Darlington pair T3 and T4 to energise coil 24A and close valve 24 during the expiratory period. Similarly when T2 is non-conducting valve 25 is closed by energising its operating coil 25A during the inspiratory period.

When T2 conducts a voltage is developed across RE2 and charging current for capacitor TC flows through REl, thereby developing a voltage of such polarity as to bias the emitter of T1 beyond cut-off, the base of T1 being at a potential determined by the current flowing in the potentiometer comprising resistor R13, pre-set resistor R14 and resistor R10. As the charging current decreases exponentially with time the emitter voltage of T1 will also fall until T1 starts to conduct and a cumulative action due to the positive feedback switches T2 off with T1 conducting for a further period of time. The time taken for the emitter voltage of T1 to fall to the value required to initiate the switching action is thus determined by the value of R151, and this may be altered by means of a knob, calibrated from 1/2 to 5 seconds, controlling the setting of potentiometer R3 and the duration of the expiration period. In a like manner a similar knob controls potentiometer R6 and the inspiration period, resistor R15, preset resistor R16 and resistor R8 controlling the potential of the base of T2. Nominally equal resistors R1 and R4 and preset resistors R2 and R5, in conjunction with preset resistors R14 and R16, enable correct matching of actual times to knob calibration despite variations in the parameters of transistors T1 and T2 and in the capacity of TC.

Visible through a suitable aperture in the panel carrying the controls of potentiometers R3 and R6 is an indication of respiration rate. This utilizes the principle disclosed in British Patent No. 914352 and employs overlapping plates operating in slides and driven by rack and pinion gearing from the control shafts of potentiometers R3 and R6. An interlock is arranged such that the expiration period is never less than the set inspiration period, though the former may be increased by operation of the shaft controlling R3 without affecting the time period set by the inspiration control shaft of R6. The indication of respiration rate shown is only applicable when the ventilator is time cycled.

Resistor R17 and R18 of low value are arranged to be connected by switches in parallel with RBI and RE2 respectively as required. Such connection in a circuit through which the charging current of the capacitor TC is flowing shortens the time constant involved and rapidly, e.g. in less than one tenth of a second switches from one phase of the respiratory cycle to the other.

Shunting each operating coil 24A and 25A are normal means (not shown) to reduce inductive effects caused by switching the said coils, the resistor R19 and decoupling capacitor 119 preventing any switching transients affecting the timing curcuit. The shunting means or other methods may be arranged to slow the opening of valves 24 and 25 to give a required amount of overlap, i.e. both valves simultaneously closed, where this is required. Resistors R20, R21, R22, R23 are emitter resistors of the Darlington pairs, T3, T4 and T5, T6, some of these resistors being common to more than one transistor.

A single pole single throw trigger selection switch 120 may be closed to insert R17 in parallel with REl when contacts 89A of the negative pressure switch 89 are closed. Thus should a patient attempt to inspire during an expiratory period and so produce a negative pressure below that set by the calibrated control knob of pressure switch 89, the expiratory period will be ended immediately and an inspiration period started.

A double pole-single throw pressure selection switch 121 may be closed enabling contacts 89A or contacts 90A of the positive pressure switch 90 to introduce resistors R17 or R18 in parallel with REl or RE2 respectively to terminate the expiration and inspiration periods respectively in pressure cycling. Depending on the settings of the time control potentiometers R3 or R6 the pressures set by the controls on the pressure switches 89 and 90 and the setting of the negative control valve 91, the ventilator will function by time cycling or pressure cycling, or a combination of both, both inspiration and expiratory periods being controlled by the time set unless the set positive and negative pressures are first attained. When no negative pressure is employed during pressure cycling, the expiratory phase will thus be time controlled.

A double pole-single throw volume selection switch 122 may be closed, one pole of which enables the closure to the normally open contacts of micro switches 105 and 109 to effect the introduction of resistors R17 and R18 respectively. Operation of the knobs controlling the masking plates 103 and 104 alters the position of these micro switches relative to the striker bar moving in accordance with the expansion and contraction of the bellows, and providing the necessary volumes are attained to close the micro switches prior to the set times, the machine functions as a volume cycled ventilator, the closure of switch 105 ending the expiration stroke and the closure of switch 109 ending the inspiration stroke. In a similar manner to that described for pressure cycling, any combination of time and volume may control the respective periods of the respiratory cycle.

Closure of both switches 121 and 122 establish a condition in which either time, pressure or volume may terminate the inspiration and the expiration periods. Under these conditions the set times will control the respirator unless either the set volumes or set pressures are attained in a shorter time.

When under the control of the electronic circuit a malfunction of the ventilator or its associated connections with the patient or an abnormality of the respiration or respiratory system of the patient will result in an expansion or contraction of the bellows beyond the normal range of movement, and this is arranged, to operate a remote monitor unit giving visual and/or audible indication of such malfunction or abnormality.

During time and/or pressure cycling of the ventilator the remote monitor unit may be activated by the striker bar 100 opening either one of the serially connected normally closed contacts of the monitor micro switches 106 and 110. The circuit of the remote monitor unit employs a bridge type rectifier circuit associated with a relay normally held energised as in British Patent No. 970626 The monitor switches 106 or 110 are opened by either face 108 or face 112 of the striker bar 100 contacting the operating arm of the associated monitor switch 106 or 110. As the position of these monitor micro switches relative to the striker bar 100 may be varied under the control of the knobs operating the masking plates 103 and 104, the amount of expansion and contraction of the bellows to operate the monitor unit may be set by the saidmasking plate controls. Under conditions which do not activate the monitor, the pointer 101 will sweep an area of aperture 102 equal to or less than that exposed by the masking plates, an indication being given by the monitor should the pointer fractionally mask or travel beyond an end of a masking plate visible in the aperture 102.

When volume cycling is introduced by switch 122, control of expansion and contraction of the bellows is by means of micro switches 105 and 109 the excursions of the bellows and pointer 101 being controlled by the masking plates 103 and 104. An abnormality or malfunction will cause the bar 100 to travel beyond a volume switch and operate one or other of the monitor switches 106 or 110 as in time and pressure cycling. Due to the relatively close operating positions of the paired volume and monitor switches (governed by the relative positions of the operating faces 107, 108, 111, and 112) and to the fact that in some medical applications it is desirable to completely empty an inspiration bellows for a short period of time, a monitor switch may possibly be opened during normal volume cycling. To prevent such opening giving an immediate indication of slugging condenser SC serially connected with a resistor R24 is introduced by a second pole of switch 122.

A double pole-double throw switch 123 is used, as in British Patent No. 970626 to set or re-set the monitor unit, and momentary closure of a single pole-single throw switch 124, spring loaded to the open position, is provided to silence an audible indicator in the monitor unit. A visual indicating lamp 125 is illuminated when the monitor is activated and this remains illuminated when the audible indicator is silenced by switch 124. A resistor R25 prevents damage to and/or rapid discharge of the battery 113 should there be a condition on the remote monitor unit or connecting wires which would in the absence of the resistor R25 present a low resistance path across the battery. Leads connecting the monitor unit to the ventilator terminate in a plug mating with the terminals 128, 129, 130, 131, and 132. Two other lamps 126 circuit 127 indicate respectively when illuminated that power from the electric mains is applied ON/OFF the ventilator and that OFF position battery therein is fully charged.

FIGURE 3 in which all relays are shown unenergised shows the circuit of the remote monitor units, broken lines leading thereto indicating connecting leads to the ventilator. The above described components of the monitor circuit are also shown at the left of the circuit. A double pole-double throw ON/OFF switch 133 in the OFF position breaks by a pole 134 the negative supply from the ventilator battery 113 via terminal 132 and protective resistor R29, and by a pole 135, a negative supply from a bridge rectifier 136 when mains voltage is applied to the primary of a transformer 137 a secondary ofwhich feeds the bridge rectifier 136. With switch 133 closed and mains voltage applied to the primary via a protective fuse 138 a relay Z/2 is energized. Contact 21 moves from the unenergized position shown, removing the ventilator battery voltage from conductor 139 and replacing it by the rectified voltage. Assuming that a monitor relay X/Z, slugged to give slow release, is energised, contact X1 is closed and the relay X/2 obtains its supply from the negative terminal of the battery via the closed monitor micro switches 110 and 106, one pole of switch 123, resistor R25, terminal 129, a press to test switch 140 which is spring loaded to the closed position, contact X1 and diode D1. The return circuit to the positive terminal of the battery is via diode D2, terminal 128 and the second pole of switch 123. Contact X2 is operated and connects conductor 139 to conductor 141 to illuminate the lamp 142 via closed contact 22 to indicate that mains are applied and that the unit is functioning normally. lfthe mains supply is interrupted relay Z/2 is deenergised, battery power is applied to conductor 139 and lamp 142 is extinguished by contact 22 opening. Should the negative supply to relay X/Z be interrupted by the opening of any of switches 110, 106 or 140, contact X1 will open and prevent subsequent closure of the opened switch energising relay X/2, and contact X2 will supply power to conductor 143 to illuminate a visual indicator lamp 144 and, via conductor 147A and a closed contact Y1 of an unenergised relay Y/l, energise a buzzer 145. During volume cycling the slugging condenser SC will discharge through the relay X/Z to hold it operated after such opening of switch 106 or for a period of time determined by the resistance of the discharge path to prevent immediate indication being given. Resistor R24 both increases the resistance of the discharge path and serves as a protection to the battery 113 should a short circuit develop in the slugging condenser SC. The condenser SC, resistor R24 and the second pole of switch 122 may be omitted providing that the faces of the striker bar operating the various micro switches are adequately spaced. Provision is made by terminals TT for connection of additional warning devices. Power is also passed via a protective fuse 146, and terminal to illuminate lamp 125 in the ventilator. The activated buzzer may be de-energised and silenced at the remote monitor unit by briefly operating switch 147, spring loaded to the position shown, and so applying power from conductor 143 to energise relay Y/1. Contact Y1 then changes over to remove power from buzzer 145 and apply it to relay Y/l which is thus held energised when switch 147 is released. With switch 147 not operated, spring loaded switch 124 at the ventilator performs a similar function by, when operated, completing a circuit between conductors 143 and relay Y/l via fuse 146 and terminals 130 and 131.

To energise or re-energise relay X/2, i.e. to set or re-set the monitor for use, switch 123 is briefly operated. During operation the polarity of terminals 128 and 129 are reversed and current flow through the relay, in the same direction as previously described, passes through diodes D3 and D4. Due to relay X/2 being slow to release, the interval between transfer of polarity when switch 123 is released is too small for contact X1 to open, and the subsequent re-established normal polarity holds the relay energised as previously described.

An alternative form of monitor 148 such as described in British Patent Application No. 46661/65 may be utilised, this either being built into the ventilator as shown in FIGURE 1 by the pipe indicated by the broken line 149 or adapted to be interposed between the port 32 and tube 42 as indicated by the broken line 150. The remote monitor unit and parts of the monitor circuit shown in FIGURE 1 and repeated on the left of FIGURE 2 are then redundant.

In order that the controls affecting the duration of the inspiration and expiration periods of only one method of cycling (time, volume or pressure) may be operative at any one time an alternative switching arrangement may be utilised and FIGURE 4 shows the relevant parts of FIGURE 2 suitably modified to achieve this.

Six four-position switches 151, 152, 153, 154 and 156. operated by a common control shaft and knob are employed, the four sequential positions representing Off, Time, Volume and Pressure. Switches 151 and 152 take the place of the two poles of switch 115, with switch 151 making the connection to the lead 116 in the OFF position shown. In the other three positions these switches switch power from the battery 113 and/or charger 114 to the ventilator. Switch 153 is placed in the emitter circuit of transistor T1 and completes this circuit by selecting, in the Time position the resistors R2 and potentiometer R3, in the Volume position the resistor R26 providing a longer time constant than the maximum in the Time position, and in the Pressure position a potentiometer R27. Switch 154 is similarly placed in the emitter circuit of T2 and introduces either resistor R5 and potentiometer R6 for Time or resistor R28 for Volume and for Pressure. Switch 155 takes the place of the second pole of switch 122 and introduces resistor R24 and condenser SC as previously described during Volume cycling. Switch 156 takes the place of both the second pole of switch 121 and the first pole of switch 122. In the Off and Time positions switch 156 makes no connections, but in the Volume position it enables micro switches 109 and 105 to bring resistors R18 and R17 into circuit. In the Pressure position, it allows pressure switch contacts 90A to bring resistor R18 into circuit. With no negative pressure in use during pressure cycling, the expiratory cycle is time controlled by potentiometer R27, the resistance in circuit being adjusted by a calibrated control knob. When negative pressure is in use the negative pressure switch contact 89A may be brought into use by the Trigger switch 120, the respective settings of the controls of the negative pressure switch 89 and potentiometer R27 determining which terminates the expiration period.

For safety reasons when employing explosive anaesthetic gases all parts shown in FIGURES 2 and 4 except references 89A, 90A,105,109,106,1l0,1l3,l14, 24A, 25A and 128 to 132 are enclosed within a compartment one wall of which forms a control and indicating panel, on which the associated controls and indicators are mounted, for operating the ventilator. This panel forms part of the cladding of the framework. Electrical connections to the compartment are by plug and socket and the whole compartment is easily removed from the ventilator for replacement or servicing. The circuits associated with switches not enclosed in the sealed compartment i.e. 89A, 90A, 105, 109, 106 and 110, are oflow voltage and relatively high resistance to reduce the risk of explosions when explosive gas is in use, while the gas circuits employ antistatic rubber bellows, tubes and pipes with the associated metal parts bonded to the framework, which is supported on metal feet.

In addition to the provision of valve 95 in center bellows 21 to prevent excessive pressure build up, safety valves, lifting at predetermined adjustable pressures are incorporated in the ports 29 and 30 and in the pipe 66 connecting the reservoir 65.

We claim:

1. A medical respirator comprising a first bellows, a second bellows and a third bellows, link means interconnecting the three bellows whereby they expand and contract in unison, biassing means tending to collapse each of the bellows, first conduit means connecting a first gas port to the first bellows, second conduit means connecting the first bellows to atmosphere, an inspiratory control valve in the second conduit means, third conduit means connecting a second gas port to the second bellows, said second gas port being connected to a gas reservoir, fourth conduit means for connecting the second bellows to a patient, fifth conduit means for connecting the patient to the third bellows, an expiratory control valve in the fifth conduit means, sixth conduit means connecting the third bellows to a third gas port connected to the gas reservoir and control means, which control means includes switching means connected to the inspiratory and expiratory control valves for switching the said valves from an inspiratory period to an expiratory period and vice-versa, an adjustable timing device connected to the switching means and arranged to control the switching means to control the inspiratory and expiratory periods of the respirator in dependence on time, a volume responsive device connected to the said link means for detecting the volume of gas passing to and from a patient and arranged to control the switching means in dependence on the volume of gas passed to and from the patient, and pressure responsive means connected in the said fourth conduit means for detecting the pressure of gas passing to and from the patient and arranged to control the switching means in dependence on the pressure of gas fed to and from the patient.

2. A medical respirator as claimed in Claim 1 in which selection means is provided for selectively connecting the pressure responsive devices and the volume responsive devices to the switching means.

3. A medical respirator as claimed in Claim 1 in which said second and third gas ports are openable to atmosphere and further comprises valve means for connecting a patient gas supply port to the said first bellows through at least part of said first conduit means, further valve means for connecting the first bellows to a patient through at least part of said second conduit means, the inspiratory control valve and at least part of said fourth conduit means, and further valve means for connecting the second bellows to atmosphere both through at least part of said third conduit means and through at least part of said fourth conduit means.

4. A medical respirator as claimed in Claim 3 which includes a manually controllable gas receiver and a one way valve from the patient to atmosphere and in which further valve means are provided for connecting the manually controllable gas receiver directly to said fourth conduit means for supplying gas to the patient.

5. A medical respirator as claimed in Claim 4 in which a single control shaft is provided connected to all the said valve means for adjusting the respirator for closed circuit ventilation, open circuit ventilation and manual ventilation.

6. A medical respirator as claimed in Claim 1 in which the timing device comprises an electronic time control circuit.

7. A medical respirator as claimed in Claim 1 in which the timing device includes an electronic timing circuit and the pressure responsive means and the volume responsive device include electrical switch contacts connectable to the said circuit, said circuit being contained in a housing sealed from the gas in the respirator.

8. A respirator as claimed in Claim 7 in which the electronic timing circuit includes two resistance chains each by-passed by a circuit including one of the said electrical switch contacts, whereby closure of one of the said electrical switch contacts decreases the effective resistance value of one of the resistance chains and causes the electronic timing circuit to operate said switching means thereby terminating a period of -a respiratory cycle before it would normally be terminated by the electronic timing circuit if said electrical switch contact was not closed.

9. A medical respirator as claimed in Claim 1 in which the link means comprises a movable platform and in which the biassing means comprises torsional spring means arranged to operate on the said movable platform to provide a force to return the bellows to a collapsed condition when the inspiration control valve is open.

10. A respirator as claimed in Claim 1 in which the third bellows has a greater stroke volume than that of either of the two other bellows to produce a negative (below atmospheric) pressure to assist in extracting gas from the lungs of a patient, in combination with a variable orifice for adjusting the negative pressure between desired limits.

1]. A medical respirator as claimed in Claim 10 in which a safety valve is connected to the third bellows to avoid the possibility of excessive negative pressure being developed to the detriment of a patient. 

26. GAS MAY BE SUPPLIED TO AND REMOVED FROM A PATIENT ON OPEN OR CLOSED CIRCUIT OPERATION OR ON OPERATION OF THE MANUAL BAG. THE INSPIRATION AND EXPIRATION CYCLE IS BY AN ELECTRONIC TIMING CIRCUIT, PRESSURE RESPONSIVE MEANS (89,90) AND VOLUME SENSITIVE MEANS (100). THE CYCLES WILL BE DETERMINED BY TIME UNLESS THE PRESSURE OR VOLUME OF GAS FED TO OR FROM THE PATIENT REACHES PREDETERMINED LIMITS BEFORE THE EXPIRATION OF THE TIME ALLOWED. 